The COSHH Essentials tool implemented a process described in a document called, if I remember correctly,  the Technical Basis of COSHH Essentials, which as far as I know you can still get on the HSE website.  This document essentially gives an algorithm for the assessment. So you start with a table in which you can look up which hazard codes give you which hazard classes, and then there are tables about vapour pressure and quantities. It is possible but laborious to follow it just as it is (I have done this).  Someone with coding skills would quite easily be able to replicate the tool.

No, I don't - but I wouldn't be surprised if someone has.

There have long been paid-for online "COSHH assessment" systems that do much the same thing but with padded out, colourful outputs and sold with hyped up claims about compliance.  They don't really do much more than the free tool did, but they try to give every appearance of being a whizzy way to guarantee COSHH compliance (which of course they can't).